OFDM (Orthogonal Frequency Division Multiplexing) is a transmission scheme used in a number of applications including digital audio broadcasting and digital TV systems (e.g. DVB-T (Digital Video Broadcasting-Terrestrial), DVB-H (Digital Video Broadcasting-Handheld) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial)).
The bit stream that is to be transmitted is split into several parallel bit streams, typically hundreds or thousands. The available frequency spectrum is split into several channels and each low bit rate stream is transmitted over one channel using some sort of known modulation scheme e.g. QAM (Quadrature Amplitude Modulation) or PSK (Phase Shift Keying). The symbols have relatively long duration so as to improve the tolerance to multi-path distortion. The channel frequencies are chosen such that the modulated data streams are orthogonal to each other. This means that each channel can be deciphered independently at the receiver, since cross-talk between the sub-channels is eliminated.
In practice, each of the sub-channels may be distorted by the transmission channel such that the amplitude and phase of each sub-carrier must be equalized in the receiver to give good performance using coherent demodulation. The receiver needs a good estimate of the transmission channel in order to carry out equalization. In order to deal with this in the digital TV systems mentioned above, scattered pilots are inserted at regular intervals across the frequency span of each symbol.
Each pilot is a symbol transmitted with known amplitude and phase and the pilots are used for channel estimation in the receiver. In the particular transmission schemes discussed above, every 12th sub-carrier (in the frequency direction) of each symbol is a pilot.
In order to increase the effective sampling frequency of the channel (in the frequency direction), the pilot sampling grid is advanced by three sub-carriers on every consecutive symbol in time. This leads to a pilot sampling grid in the time-frequency plane as shown in FIG. 1.
In FIG. 1, time (i.e. symbol number n) is shown on the y-axis. The oldest symbol is at the top of the plot (symbol number 0) and the most recently received symbol is at the bottom of the plot (symbol number 15). Frequency (i.e. sub-carrier k) is shown on the x-axis. Typically, there will be many more sub-carriers per symbol than are shown in FIG. 1. As indicated by the key of FIG. 1, each sub-carrier is shown by a dot and the scattered pilot sub-carriers are shown by a dot overlaid with a rectangle.
Note, in FIG. 1, that some sub-carriers, such as sub-carrier index k=0, are designated continuous pilots and as such are transmitted as a known pilot for every symbol n.
Channel estimation at the receiver usually uses the scattered pilots. In the process, the receiver aims to form an estimate of the distortion applied by the channel for each sub-carrier of each symbol received. Ultimately, the receiver must form a channel estimate for each active data sub-carrier of the symbol, and then it must use that channel estimate to equalize the received OFDM data.
A conventional DVB-H receiver follows the steps shown in the flowchart of FIG. 2 to process each OFDM symbol. Input samples are received and output data is produced.
At step 201, the Fast Fourier Transformer (FFT) takes the well-timed OFDM symbols and transforms them from the time domain to the frequency domain by applying a Fourier transform.
At step 203, the pilots are extracted from the symbol. Both continuous pilots and scattered pilots may be used to assist the receiver to estimate the channel for each sub-carrier position.
At step 205, channel estimation is performed. Information, typically from the inserted pilots, is used to derive an estimate of the distortion produced by the channel for each active sub-carrier of the OFDM symbol. As already mentioned, for an OFDM receiver, the channel estimate is typically a single complex value per active sub-carrier.
At step 207, the channel estimate is inverted and the equalizer uses this to cancel, as best as possible, the distortion introduced by the channel and restore each OFDM sub-carrier to that which was produced in the transmitter.
At step 209, the demapper uses the result of the equalizer along with knowledge of the modulation scheme used (e.g. QPSK, 16-QAM or 64-QAM) and channel state information to produce a number of soft decisions for each sub-carrier.
At step 211, de-interleaving is performed. The de-interleaver does the scrambling across frequency that was applied by the interleavers of the transmitter. This helps to spread localized frequency distortions, which improves the performance of the Viterbi decoder to typical impairments.
At step 213, the Viterbi decoder estimates the most likely path through a trellis using the likelihood information in the soft-decisions and knowledge of the convolutional code that was used in the transmitter.
At step 215, the outer de-interleaver spreads information across time to reduce the effect of distortions that are localised in time. The purpose of the outer de-interleaver is to de-correlate the error bursts at the output of the Viterbi decoder. This ensures that the Reed-Solomon decoder is not overwhelmed by a large number of sequential errors from the Viterbi decoder.
The final step 217, is Reed-Solomon decoding. The Reed-Solomon decoder provides an additional level of error correction for any residual errors that are not corrected by the Viterbi decoder.
There are a number of techniques that may be used to form a channel estimate at step 205 of FIG. 2.
In a first technique, the received pilot sub-carriers may be interpolated (upsampled) along the frequency axis. (In the transmission systems mentioned above, this would comprise interpolating the scattered pilots by a factor 4 in the frequency direction. This will give a line of virtual pilots at every third sub-carrier in the frequency direction. These resulting virtual pilots can be interpolated by a factor 3 in the frequency direction to give a sample of the channel response for every sub-carrier.) The pilot sub-carriers that may be used for such interpolation are shown in FIG. 3.
Alternatively, the received pilot sub-carriers may be interpolated along the time axis. (In the transmission systems mentioned above, this would comprise interpolating the scattered pilots by a factor 4 in the time direction. This will give a line of virtual pilots at every third sub-carrier in the frequency direction. These resulting virtual pilots can be interpolated by a factor 4 in the frequency direction to give a sample of the channel response for every sub-carrier.) The pilot sub-carriers that may be used for such interpolation are shown in FIG. 4.
Alternatively, a channel estimate may be formed by data-directed techniques that use information from both the pilot sub-carriers and the data sub-carriers.
The various techniques for channel estimation (those listed above and others) each have different strengths and weaknesses, and one may perform better than the others, given a particular type of channel distortion.
OFDM receivers for the TV standards mentioned above (DVB-T, DVB-H and ISDB-T) are required to operate in a wide variety of channel impairments e.g. additive noise, highly frequency selective channels, high levels of Doppler shift causing rapidly changing channel conditions and inter-carrier interference (ICI), and high levels of co-channel and adjacent channel interference.
A particular OFDM receiver may achieve acceptable performance in all the different conditions (channel impairments) using a design of equalizer that is a compromise for all types of channel impairment. However, for any one set of channel conditions, a receiver may achieve better performance if it uses an equalizer that is specifically designed to cope with the particular channel conditions.
In one example, an equalizer may be designed to use interpolation of pilots along the time axis using a delay store of received pilots from a number of consecutive symbols (see FIG. 4). An equalizer like this might give good performance at low Doppler frequencies over a wide range of delay spreads, but it will give poor performance if the Doppler frequency is high.
In another example, an equalizer may be designed to use interpolation of pilots along the frequency axis using received pilots on one symbol (see FIG. 3). An equalizer like this might give good performance for a channel with low delay spread over a wide range of Doppler frequencies, but it will give poor performance if the delay spread is high.
Thus, a problem associated with known receivers is that optimal receiver performance cannot be achieved across a wide range of channel conditions.
It is an object of the invention to provide a method and apparatus which avoids or mitigates the problems of known systems described above.